PROJECT SUMMARY/ABSTRACT Advanced age is associated with an increase in likelihood of having presbycusis, a hearing impairment treated primarily through the use of hearing aids (HA) and aural rehabilitation. With twenty percent of the population predicted to be 65 years or older by 2030, hearing loss is a public health concern. A significant problem is that the prevalence of HA use among people with hearing loss is low, and untreated hearing loss contributes to a reduced quality of life, avoidance of social situations, and feelings of loneliness. The most common shortcoming of HA is poor performance in noisy environments, but traditional standard-of-care audiometry does a poor job explaining why this occurs. Many factors beyond the audiogram have been suggested as possible explanations for poor speech understanding in noise; however, none have explained a substantial portion of the variance in outcomes until recently. Both hearing loss and age cause declines in the ability to use fine temporal and spectral cues. Recent evidence suggests that sensitivity to joint spectro-temporal modulations (STM; i.e., spectral patterns that change over time, as occur in natural speech sounds) explains a significant portion of the variance in speech understanding in noise, after controlling for audibility. Notably, STM is also predictive of self-reported outcomes. Individual differences in STM sensitivity, combined with the known effects that HA processing has on distorting spectro-temporal cues, mean that some HA users may be set up for failure. It is hypothesized that successful HA use partially depends on a person?s ability to process spectro-temporal signals and the integrity of the cues (distorted by HA processing) delivered to the listener?s auditory system. The long-term goal is to optimize a novel evidence-based, HA-fitting intervention, by building on a strong set of preliminary data showing that sensitivity to STM is a robust predictor of speech understanding in noise and self-reported outcomes for HA users. The first aim is to comprehensively describe the nature of the interaction between HA distortion and STM sensitivity and subsequent effects on aided outcomes using an observational approach. Under this aim, validation of suspected underlying mechanisms responsible for STM sensitivity will also be examined. The second aim is to determine the extent to which altering spectro-temporal cues, through HA manipulation, affects speech in noise performance, using a randomized, cross-sectional design. Other patient-centered variables will be assessed, such as working memory, age, and audibility, which may be responsible for remaining variability in outcomes. This work translates basic hearing science principles into clinical applications, and will have implications for individualizing existing clinical treatments.